The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed the Global System for Mobile communications (GSM). Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs.
FIG. 1 illustrates a cellular network with a serving node 101 that serves a UE 103 located within the serving node's geographical area of service, called a cell 105, Depending on the system, the serving node 101 may e.g. be a base station, a Node B, or an evolved Node B (eNodeB or eNB). Hereinafter, the serving node 101 will be referred to as an eNB in the non-limiting example of an LTE system.
Relay Nodes
Relay Nodes (RNs) are one of the earliest proposals to extend the coverage of cellular networks. Apart from this, RNs could also help to enhance capacity in hotspots and to increase the effective cell throughput. Also, the average radio-transmission power at the UE could be greatly reduced, especially in highly shadowed areas, thereby leading to longer UE battery life.
It is due to the aforementioned advantages of relaying that LTE-Advanced, the standardization of which is currently being finalized in 3GPP, has introduced support for RNs. The LTE-Advanced standard corresponds to Rel-10 of LTE.
An RN cell, as specified in Rel-10, appears to a UE as a separate cell distinct from the donor cell. The RN cells have their own Physical Cell Identity (PC) as defined in LTE Rel-8 and transmit their own synchronization channels, and reference symbols. The UE receives scheduling information and Hybrid Automatic Repeat-reQuest (HARQ) feedback and other control signaling directly from the RN and sends its control channels to the RN. A type I RN appears as a Rel-8 eNB to Rel-8 UEs, i.e. it is backwards compatible. This means basically that from a UE perspective, there is no difference being served by an eNB or a type I RN.
FIG. 2 illustrates a RN 204 with a service area or cell 207, the RN 204 communicating with a so called donor eNB (DeNB) 202 with a service area or cell 206, and one or several UEs 203 located within the RN's cell 207. Transmissions between UE 203 and RN 204 are done over a radio interface denoted Uu, which is the same as for regular eNB to UE communication. Transmissions between the RN 204 and the DeNB 202 are made over a radio interface denoted Un, which reuses much of the functionality of the Uu interface. This means that the DeNB 202 handles the RN 204 as a UE. The DeNB thus provides backhaul transport for the RN and all the UEs connected to the RN. The signaling and the radio protocols used on the Un interface are based on the Uu interface of the LTE Rel-8 standard with only small additions and modifications. An overview of the relay support in LTE Rel-10 is described in 3GPP TS 36.300, chapter 4.7.
FIG. 3 illustrates the overall Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 300, which is the radio network of the LTE system architecture for support of RNs. The RN 304 terminates the S1/X2 interfaces in the same way as a normal eNB 301. The S1 interface is however not directly connected to the Mobility Management Entity and/or Service Gateway MME/SGW 308a-b as for normal eNBs 301, but instead the S1 control messages and data are forwarded between the RN 304 and the S1 interface associated with the DeNB 302. The RN 304 has an S1 interface setup towards the MME/SGW 308a-b, which is proxied in the DeNB 302. The RN 304 may also have an X2 interface setup towards other eNBs 301, in which case the X2 interface is proxied in the DeNB 302.
The S1 user plane protocol stacks for supporting RNs are shown in FIG. 4a. There is a GTP tunnel associated with each UE Evolved Packet System (EPS) bearer, spanning from the SGW 408 associated with the UE to the DeNB 402, which is switched to another GTP tunnel in the DeNB 402, going from the DeNB 402 to the RN 404 in a one-to-one mapping.
Similarly, the X2 user plane protocol stacks for supporting RNs during inter-eNB handover are also proxied via the DeNB 402, as shown in FIG. 4b. There is a GTP forwarding tunnel associated with each UE EPS bearer subject to forwarding, spanning from the other eNB 401 to the DeNB 402, which is switched to another GTP tunnel in the DeNB 402, going from the DeNB 402 to the RN 404 in a one-to-one mapping.
The user plane packets are mapped to radio bearers over the Un interface. The mapping can be based on the QCI associated with the UE EPS bearer. UE EPS bearer with similar QoS can be mapped to the same Un radio bearer.
Mobile RNs
Providing high throughput and short handover interruption time for UEs in scenarios where several UEs are traveling at high speed together, for example in trains and buses, is challenging and it can cause signaling overhead over the air interface. Also, the possibility of handover failures increases as many almost simultaneous HO requests are initiated to a given neighboring cell. Mobile RNs, also called mobile relays, are one of the proposed solutions for solving the problem of high speed group mobility in e.g. mobile public transport, as illustrated in FIG. 5.
As shown in FIG. 5, mobile RNs 509a-b are installed on top of trains and buses, and UEs inside the moving vehicles are connected to these RNs instead of externally installed eNBs 501 or fixed RNs 504. As long as a UE is inside the moving vehicle, it doesn't have to change its serving node, which is the mobile RN 509a or 509b, no matter how many cells the vehicle traverses during the journey. Instead, the RN 509a/b will be handed over from one DeNB to another and through that process, the traffic and signaling for all UEs connected to the mobile RN 509a/b will be transferred from the core network to these UEs via another DeNB. Since in this case only the RN 509a/b is handed over between DeNBs, compared to several individual UEs 503 being handed over between eNBs, the radio signaling may be optimized in accordance with the so-called group mobility.
To enable the operation of a mobile RN, a re-assignment of the DeNB is required. This process can be accomplished in a manner similar to a UE handover, where the mobile RN is sending measurement reports of the DeNB cells it can hear to the currently serving DeNB. The measurement reports are evaluated to determine when the RN should be relocated to a neighbor DeNB. In LTE Rel-10, only static or fixed RNs are supported, but mobile RNs are a possible addition in Rel-11.
Network Management Architecture
A network management architecture is illustrated in FIG. 6, describing the situation when the RN and the eNB are from different vendors. From the top, the network is managed by the Network Management (NM) system 601 handling equipment from all vendors via the standardized Itf-N interface. Equipment from each vendor is handled by one or several Domain Management (DM) systems 602a-b, sometimes stated to include Element Management (EM). Such systems are also referred to as Operations And Maintenance (OAM or O&M) systems, or Operations and Support Systems (OSS) 602a-b. These systems manage the Network Elements (NE) 603a-c which can be eNBs and RNs, and also other elements.
In LTE, two eNBs, 603a and 603b, can be interconnected via the X2 interface for information exchange. RNs in the network are controlled, just like any UE, by the Radio Resource Control (RRC) protocol, which has been extended with certain relay-specific functionality. Bearers are also set up to carry X2 and S1 interfaces between RN and its donor eNB, as well as to carry O&M traffic between RN and its DM/EM. If the eNB and the RN are from the same vendor, both nodes can be managed by the same DM/EM.
Automatic Neighbor Relations
The PCI is an essential configuration parameter of a radio cell. PCIs are grouped into 168 unique physical layer cell identity groups, each group containing three unique identifies. Thus, there are only 504 different Pas altogether. Limiting the number of Pas makes the initial PCI detection by the UE during cell search easier, but the limited number of PCIs inevitably leads to the reuse of the same PCI values in different cells. Therefore, a PCI may not uniquely identify a neighbor cell, and each cell additionally broadcasts, as a part of the System Information (SI), a Globally unique Cell Identifier (CGI).
When a new node, such as an eNB or a RN, is brought into the field, a PCI needs to be selected for each of its supported cells, avoiding collision with respective neighboring cells. The use of identical PCI by two cells in close proximity results in interference conditions that might hinder the identification and use of any of them. Otherwise, if both cells have a common neighbor, handover measurements that are based on PCI will become ambiguous thus leading to confusing measurement reports or even to the handing over of a UE to the wrong cell, which can cause Radio Link Failure (RLF).
The PCI assignment shall fulfill the following two conditions:                1. Collision-free: The PCI is unique in the area that the cell covers.        2. Confusion-free: a cell shall not have more than one neighboring cell with identical PCI.        
Using an identical PCI for two cells creates collision, which can only be solved by restarting at least one of the cells and reassigning Pas upon restart, causing service interruption. PCI confusion, on the other hand, can be resolved by instructing UEs to read the CGI of the concerned neighbor cell. UEs need to be assigned long enough idle periods, for example using DRX configuration with long DRX cycles, in order to read the CGI from the broadcast channels of neighbor cells. Therefore, putting a PCI in use which causes confusion is highly undesirable as the UE might have to be requested to decode the CGI, which can cause service interruption from the serving cell during the CGI measurement duration.
Traditionally, a proper PCI is derived from radio network planning and is part of the initial configuration of the node. The network planning tool calculates the possible PCIs for the new cell(s) based on estimated neighbor relations of the new cells, as estimated by cell coverage area predictions. However, prediction errors, due to imperfections in map and building data, and to inaccuracies in propagation models, have forced operators to resort to drive/walk tests to ensure proper knowledge of the coverage region and identify all relevant neighbors and handover regions. Even the accuracy of that is questionable as some factors such as seasonal changes, such as the falling of leaves or snow melting, can alter the propagation conditions. Also, the inaccuracy of cell coverage and neighbor relation assessment increases with time as the live network and its surroundings evolve over time.
LTE has a feature known as UE Automatic Neighbor Relations (ANR), which allows UEs to decode and report the CGI information of neighbor cells to the serving cell upon request, in addition to the PCI which is included in almost all measurement reports. eNBs maintain a Neighbor Relation Table (NRT) for each of their cells. Apart from the PCI to CGI mapping, each neighbor relation contains other relevant information such as the possibility of X2 connectivity.
The CGI of the neighbor cells are the ones that are used when signaling to the neighbor eNB via the MME, since the MME routes the messages based on eNB identity which is a part of CGI. If the policy is to establish X2 for neighbor relations and if X2 is not already available, then the CGI can be used to recover the target node's IP address, which is used for X2 setup. When the X2 interface is established, the neighboring eNBs can share information about their served cells including PCIs and CGIs. It is also possible to share such information via OAM.
Positioning
Several positioning methods for determining the location of a target device, which may e.g. be a UE, a mobile RN, or a Personal Digital Assistant (FDA) exist. Some well-known methods are;                Satellite based methods        Observed time difference of arrival (OTDOA)        Uplink time difference of arrival (UTDOA)        Enhanced cell ID        Hybrid methods        
The above methods are briefly described below.
Satellite Based Positioning Methods
Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that enable the target device to locate their position. Another generic term currently used in the literature for satellite based positioning method is Galileo and Additional Navigation Satellite System (GANSS). Among others, the global positioning system (GPS) is the most well-known example of GNSS, which is currently in operation and has been so for more than a decade.
The assisted GNSS (A-GNSS) or assisted GPS (A-GPS) is tailored to work with the target device, and thus enables the device to relatively accurately determine its location, time, and even velocity including direction in an open area environment, provided a sufficient number of satellites are visible. Among various positioning methods, A-GPS is considered to be one of the most viable and commonly used one. The A-GPS can be UE based or UE assisted. In both cases the network node, e.g. the eNB, sends assistance data such as satellite information to the target device to facilitate the GPS measurements. However in UE based A-GPS, the target device or UE reports measurements to the network which in turn determines the location of the device. In the latter case the target device itself finds its location based on assistance data and measurements.
OTDOA
In OTDOA, the target device measures the timing differences of downlink Positioning Reference Signals (PRS) received from multiple distinct locations e.g. eNBs. For each measured neighbor cell, the UE measures RSTD. The RSTD is the relative timing difference between a neighbor cell and a reference cell. The UE position estimate is the intersection of hyperbolas corresponding to the measured RSTDs. At least three RSTD measurements from geographically dispersed base stations with a good geometry are needed to accurately determine the UE location coordinates. In UE assisted OTDOA method, the UE sends the RSTD measurements to the positioning node, which in turns uses the RSTD measurements to determine the UE location. In UE based OTDOA method, the UE performs RSTD measurements as well as determine its location.
In both UE based and UE assisted OTDOA, the positioning node, for example the Evolved Serving Mobile Location Center (E-SMLC) in LTE, provides assistance data to the UE to facilitate the RSTD measurements. The assistance data includes information such as cell identities, their reference timing, PRS bandwidth, and periodicity of PRS.
UTDOA
In the UTDOA positioning method, several network radio nodes, called Location Measurement Units (LMU), perform measurements on uplink signals transmitted by the target devices. The LMU is typically located at the base station, eNB or DeNB sites, but it may also be located as a separate unit in a network. The LMUs communicate with the positioning node (e.g. E-SMLC in LTE). The LMUs send measurements done on UE uplink signals to the positioning node which in turn determines the location of the target device.
Enhanced Cell ID (E-CID)
The E-CID positioning method uses the network knowledge of geographical areas associated with cell IDs and additionally one or more UE and/or base station measurement(s) to determine the location of the target device. The measurements include at least the Cell Identification (CID) and the corresponding geographical location, such as coordinates, of the serving cell. Examples of the additional measurements are:                Timing Advance (Tadv). Tadv is derived from eNB Rx-Tx time difference measurement and/or UE Rx-Tx time difference measurement.        Angle of arrival (AoA) measured at the base station.        Signal strength measurement, e.g. path loss/path gain, RSRP from serving and neighboring cells.        Signal quality measurement, e.g. RSRQ from serving and neighboring cells.        Inter-RAT measurements, e.g. CPICH RSCP, CPICH Ec/No, GSM carrier RSSI.        
A fingerprinting positioning method typically makes use of signal strength and/or signal quality such as RSRP/RSRQ. Therefore the fingerprinting method is a special type of E-CID positioning method.
Hybrid Methods
The hybrid positioning methods combine more than one positioning method to enhance the positioning accuracy of the target device. For example, the A-GNSS measurements and E-CID measurements can be used in combination to determine the location of the target device.
UE Measurements
Measurements are done by the UE on the serving as well as on neighbor cells over some known reference symbols or pilot sequences. Some measurements may also require the UE to measure the signals transmitted by the UE in the uplink.
In a multi-carrier or carrier aggregation scenario, the UE may perform the measurements on the cells on the primary component carrier (FCC) as well as on the cells on one or more secondary component carriers (SCCs). The measurements are done for various purposes. Some example measurement purposes are: mobility, positioning, Self-Organizing Network (SON) functionality, Minimization of Drive Tests (MDT), O&M, network planning and optimization.
The measurements may also comprise cell identification, e.g. PCI acquisition of the target cell, CGI or E-UTRAN CGI acquisition of the target cell, or SI acquisition of the target cell. The target cell can be an LTE or any other radio access technology cell.
Examples of mobility measurements in LTE are measurements of:                Reference symbol received power (RSRP)        Reference symbol received quality (RSRQ)        
Examples of inter-RAT mobility measurements are measurements of                Common pilot channel received signal code power (CPICH RSCP)        CPICH Ec/No        GSM Received Signal Strength Indicator (RSSI)        
Examples of well-known positioning measurements in LTE are measurements of                Reference signal time difference (RSTD)        RX-TX time difference        
The UE served by a mobile RN performs measurements on signals transmitted by the RN. In addition the UE may also be required to perform measurements on neighboring cells served by normal base stations and/or by a fixed RN.
Mobility Scenarios
The measurements described in the previous section can be used to enable UE mobility. These measurements are also applicable for a UE camped on or served by the mobile RN.
Fundamentally there are two kinds of UE mobility states:                1. Low activity state mobility e.g. cell reselection.        2. Connected state mobility e.g. handover, cell change order, RRC connection release with re-direction. RRC connection re-establishment, primary cell (PCell) change in multi-carrier system, and primary component carrier (PCC) change in multi-carrier system.        
In LTE there is only one low activity mobility state called idle state. In High-Speed Packet Access (HSPA) there are following low activity states: Idle state, URA_PCH state, CELL_PCH state, and CELL_FACH state.
Nevertheless, in any low activity state the UE autonomously performs cell reselection without any direct intervention of the network. But to some extent the UE behavior in low activity mobility state scenario could still be controlled by a is number of broadcasted system parameters and performance specification.
In HSPA the connected state is also called as CELL_DCH state since at least a dedicated channel (DCH) is in operation for at least the maintenance of the radio link quality.
The handover on the other hand is fully controlled by the network through explicit UE specific commands and by performance specification. Similarly, an RRC re-direction upon connection release mechanism is used by the network to re-direct the UE to change to another cell which may belong to the RAT of the serving cell or to another RAT. In this case the UE upon receiving the RRC re-direction upon connection re/ease command typically goes in idle state, searches for the indicated cell/RAT, and accesses the new cell/RAT. In both low activity state and connected state the mobility decisions are mainly based on the same kind of downlink neighbor cell measurements, which were discussed in the previous section.
Both Wideband Code Division Multiple Access (WCDMA) and E-UTRAN are frequency reuse-1 systems. This means the geographically closest or physical adjacent neighbor cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both WCDMA and E-UTRAN could be broadly classified into three main categories, for both low activity and connected states:                1. Intra-frequency mobility        2. Inter-frequency mobility        3. Inter-RAT mobility        
In intra-frequency mobility UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay as mobility measurements can be carried out in parallel with channel reception. In addition an operator would have at least one carrier at its disposal that he would like to be efficiently utilized.
In inter-frequency mobility the UE moves between cells belonging to different carrier frequencies but of the same access technology. This could be considered as the second most important scenario.
In inter-RAT mobility the UE moves between cells that belong to different access technologies such as between WCDMA and GSM or vice versa, or between WCDMA and LTE or vice versa, and so on.
As already mentioned above, a mobile RN will typically be deployed in a movable vehicle, such as a bus, a boat, or a train, either on the outside of the vehicle, such as on top or on the side of the vehicle, or on the inside of the vehicle. A large number of subscribers is typically located in the small area of the vehicle. Due to vehicle mobility it is advantageous that all subscribers inside the vehicle are served by the mobile RN. This can be realized provided that all the UEs belonging to these subscribers are camped on or connected to the mobile RN and not to external base stations. The vehicle housing the mobile RN may arrive at a location where there exists UEs outside the vehicle which are closer to the vehicle and the mobile RN than to an external base station. However, it may be advantageous to prevent UEs which are not inside the vehicle from camping on or connecting to the mobile RN. One reason is that external UEs camping on or connecting to the mobile RN may cause ping pong in cell selection and handovers since the mobile RN typically moves, although it may stop from time to time, and therefore moves away from the external UE. Furthermore, it may also cause a capacity problem if external UEs camps on or connects to the mobile RN, as the mobile RN has a limited capacity adapted to the possible amount of UE's in the vehicle. A scenario with similar problems as the mobile RN scenario described above is the scenario when a WiFi access point or an ordinary base station such as a pico base station or a fixed RN is used for serving a small coverage area, such as a coffee shop. In such a scenario, a user with a UE may be walking by on the street outside the coffee shop, and this may cause the same unwanted ping pong effect for handovers or cell selection as described above.
In various known approaches, the UEs inside a vehicle served by a mobile RN or in a specific coffee shop served by a base station can be connected to the mobile RN or the base station by reading a specific indicator broadcasted by the mobile RN/base station, or by means of detectors or sensors. In one example, door sensors may be used sensing when a UE is entering or leaving the vehicle or coffee shop. However such methods are not supported by the legacy UEs. Furthermore, as sensor based methods requires a complex implementation, several UEs may not support such feature even in future releases. The sensors may also be specific to e.g. a vehicle type, which means that UEs supporting such a mechanism for a specific type of vehicle may not use it in all types of vehicles.